Usually it is extremely difficult for the catalyst carrier and the catalyst used to purify the auto exhaust gas to display its purifying performance, catalyst durability and mechanical strength fully in response to the changing conditions of engine operation such as the composition, density, volume, temperature of exhaust gas or shock and vibration. Especially in the case of a conventional catalyst, as illustrated by B in FIG. 1, which is carried by transition alumina such as .gamma.-alumina, with an increase in the distance run the catalyst comes to be contaminated with a particular element of the exhaust gas or to be deteriorated through hysteresis, thereby decreasing steadily in its purifying performance. Moreover, the catalyst particles, which have a low mechanical strength, are liable to be broken and reduced to powder by vibration and shock of the vehicle, which also leads to a deterioration of the purifying performance. Meanwhile, the broken powder blocks the passage, thereby decreasing the output or a drop in the catalyst volume due to thermal contraction is liable to form an exhaust gas passage which is not related to the purifying reaction, thereby greatly reducing the catalyst durability.
Thus the conventional catalyst carrier which is mainly composed of transition alumina like .gamma.-alumina has the drawback that, since the transition alumina crystallizes at around 900.degree. C. and in consequence turns mechanically very weak, the catalyst carrier is broken with the result of the catalyst characteristic heavily deteriorating on account of vehicle vibration or thermal shock, because the mechanical strength of the transition alumina drops due to hysteresis when exposed to an auto exhaust gas of around 1000.degree. C.
Further in the case of a catalyst carrier made of transition alumina, which has a remarkably high value of specific surface area, but an extremely low value of pore volume, especially of average pore diameter, the auto exhaust gas with an extremely high space velocity suffers extremely little catalyst reaction within the carrier, the reaction predominantly taking place near the surface of the catalyst.
Therefore the catalyst is contaminated with surface deposits of specific elements in the exhaust gas, i.e., fuel and lubricant additives contained in the exhaust gas or inclusions such as phosphor or lead compounds or the combustion product of sulfur and in consequence the characteristic and durability of catalyst sharply drop as illustrated by B in FIG. 1. Thus the catalyst carrier of transition alumina has been found not always satisfactory for purifying the auto exhaust gas. This fact disproves the validity of the traditional conception that a catalyst carrier having fine pores and accordingly a large specific surface area gives the best performance in purifying the auto exhaust gas.
For the purpose of enhancing the reactivity of a catalyst carrier exposed to a reactive fluid with a high space velocity such as the auto exhaust gas, it is therefore necessary to make the reactive fluid rapidly diffuse on the internal surface of pores near the outer surface of the carrier; and to this end it is necessary to increase the average pore diameter and thereby increase the velocity of diffusion within the pores.
As the materials likely to meet this requirement, refractories with a larger average pore diameter than that of conventional transition alumina such as .alpha.-alumina, zircon, cordierite, mullite are conceivable, but they are found unavailable, because on account of their low specific surface area they are inferior in purifying characteristic, notably in the initial rate of purification, though they satisfy the conditions such as mechanical strength, heat resistance and average pore diameter. Thus with nothing ideal available for auto exhaust gas purification the demand has been strong for development of an excellent catalyst carrier and catalyst which are ideal from an anti-pollution point of view.